My tour through Jared Diamond's The World Until Yesterday has brought something interesting to light regarding the tendency to justify aggression and conquest by convenient definition of who is human and who is not. Untermenschen, unpeople: this is not just a recent European phenomenon. Primitive societies do it, too, and from this we might conjecture that the practice goes way back in history:

There are at least two other frequently offered reasons that traditional people themselves mention as motives for war. One is sorcery: it’s routine in New Guinea and many other small-scale societies to blame anything bad that happens (such as an illness or a death that we would consider natural) on an enemy sorcerer, who must be identified and killed. The other is the common view that one’s neighbors are intrinsically bad, hostile, subhuman, and treacherous and thus deserve to be attacked whether or not they have committed some specific evil deed recently. I already quoted an example for New Guinea in Chapter 3: a Wilihiman Dani man’s answer to a Dani woman about why he was trying to kill the Widaia Dani (“Those people are our enemies, why shouldn’t we kill them?— they’re not human”).

Diamond, Jared (2012-12-31). The World Until Yesterday: What Can We Learn from Traditional Societies? (p. 159). Penguin Group US. Kindle Edition.

Again:

The Wilihiman were now depressed: they had hoped to make a kill, but instead it was they who had just suffered another death. An old Wilihiman woman lamented, “Why are you trying to kill the Widaia?” A Wilihiman man replied, “Those people are our enemies. Why shouldn’t we kill them?— they’re not human.”

Diamond, Jared (2012-12-31). The World Until Yesterday: What Can We Learn from Traditional Societies? (p. 125). Penguin Group US. Kindle Edition.

Looking at these passages again, I found another matter aside from that of how we justify the violation of humane behavior. It is quite possible that their native language may not line up with ours in the aspects I am considering, but if it does we might note that the reversal of a trend we find today: in beginning-of-life debates some will argue that not all humans are persons. But here the Wilihiman man acknowledges that the enemies are people, but not humans.

I have also noticed in reading tons of kids books out loud that the various talking animals sometimes refer to each other as people. I think close attention to these passages would yield a lot about what we intuitively have in mind when we invoke "persons" in a debate.

And then there is the old theological position that angels, devils and the three members of the Trinity, are all persons, meaning possessing of intellect and will. That sense is more technical and fixed, but it further informs how we might typically, albeit subconsciously, approach the distinction between "humans" and "persons".